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Surface modification of microcrystalline cellulose: Physicochemical characterization and applications in the Stabilization of Pickering emulsions
International Journal of Biological Macromolecules 132 (2019) 1176–1184
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International Journal of Biological Macromolecules journal homepage: http://www.elsevier.com/locate/ijbiomac
Surface modification of microcrystalline cellulose: Physicochemical characterization and applications in the Stabilization of Pickering emulsions Hafiz Muhammad Ahsan, Xingzhong Zhang, Yan Li, Bin Li, Shilin Liu ⁎ College of Food Science & Technology, Huazhong Agricultural University, Wuhan, Hubei 430070, China
a r t i c l e
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Article history: Received 22 October 2018 Received in revised form 12 February 2019 Accepted 8 April 2019 Available online 9 April 2019 Keywords: MCC CMCNa Composites Molecular weight Pickering emulsions
a b s t r a c t The phenomenon of polymer adsorption was applied to modify the surface of microcrystalline cellulose (MCC). The active polymer sodium carboxymethyl cellulose (CMCNa), having different molecular weight was used to produce modified MCCs. The average particle size of unmodified MCC was about 14.208 ± 0.064 μm, and it was increased to 19.576 ± 0.26 μm after modification. The modified MCCs exhibited a typical shear thinning behavior. It suggested that the molecular weight of active polymer had an amenable and significant influence on the physicochemical properties and stability of MCCs. The composites prepared from CMCNa with high molecular weight were more stable than others. Moreover, dried MCCs could be re-dispersed in water and could be used as a stabilizer for Pickering emulsions. The obtained emulsions (CMCNa-c) showed higher stability against pH changes, ionic strength and coalescence during storage. It suggested that the re-dispersible MCCs played a significant role in stabilization of emulsions. Furthermore, the re-dispersible MCCs preserved the original properties of un-dried composites and could also be used in other food allied sectors. © 2019 Elsevier B.V. All rights reserved.
1. Introduction The phenomenon of surface modification by physical or chemical means has attracted great interests in the scientific environment due to the introduction of new possible interactions in various food ingredients. Currently, the well-established processes of surface modification includes: oxidation, acid hydrolysis, esterification, amidation, nucleophilic substitution, polymer adsorption or grafting, and so on. Interestingly, each type of surface modification had its own significance and functions that entirely depends on the core theme of application [1,2]. In the recent past, this phenomenon vigorously implemented on different materials especially on cellulosic material e.g. microcrystalline cellulose (MCC) and nano-crystalline cellulose (NCC). The selection of cellulose is mainly due to its renewable nature, large specific surface area, lightweight, low cost, and unique physicochemical properties. MCC is the most commonly used cellulose and commercially available in powder form that possesses relatively high crystallinity. Normally, MCC is produced after a mechanical disintegration of cellulosic fiber pulp through a high-pressure homogenizer [3,4]. It consists of nanocrystals (NCC), obtained from hydrolysis of cellulose fibers, either in form of parts ⁎ Corresponding author. E-mail addresses: [email protected] (H.M. Ahsan), [email protected] (Y. Li), [email protected] (B. Li), [email protected] (S. Liu).
https://doi.org/10.1016/j.ijbiomac.2019.04.051 0141-8130/© 2019 Elsevier B.V. All rights reserved.
or aggregates with the diameter of several micrometers or tens of micrometers [5]. MCC, having a different particle size and micro-fibrils networks, can be produced by using different pressure, temperature, flow rate and design of chambers of high-pressure homogenizers. It has promising applications in bio-composites, protein immobilization, drug delivery, and metallic reaction templates [6,7]. The modification of cellulosic material (NCC and MCC) has been a subject of intense research in polymer composites, such as protein, chitosan, methyl cellulose, wheat bran, and corn starch [8,9]. Over the last few years, many efforts have been performed to the employment of natural cellulose fibers as reinforcing agents in polymer composites [10]. The main reason for such modificiation is the preparation of composites that can enhance the properties of both polymers [11]. A wide variety of reactions centered on imparting new properties on the surface of cellulosic materials (MCC and NCC) along with improving the compatibility of cellulose nano-crystals with matrices for the formation of composite materials [12]. MCC can be modified with homogenization as it releases the microfibrils in the suspension or through drying. However, it leads to the formation of agglomerates and bundles in MCC due to hydrogen bonding. These structural attributes after drying create an issue to re-disperse MCC to form a homogenous suspension. The whole mechanism in this process is technically responsible for hornification or irreversible agglomeration. It is one of the major issues faced in papermaking industry
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as it is difficult to re-disperse the MCC as it forms agglomerates after drying [13,14]. To eradicate this issue, many studies were conducted and surface modification was considered as a management remedy. The phenomenon of surface modification through polymer adsorption or polymer grafting considered green technology to overcome hornification and various studies were conducted to use polymers or salts to overcome hornification [13–18]. Carboxymethyl cellulose (CMC) has the potential to act as a linker molecule when adsorbed on cellulose [14]. CMC was used in different studies because of its chemical and physical characteristics; it possesses carboxyl groups [15,16]. The adsorbed CMC improved the mechanical properties of hand-sheets prepared from wood pulp and aided in paper recycling [19]. Moreover, CMC adsorbed onto cotton fibers could increase the charge density and adsorption capacity for surfactant [20]. Owing to various physicochemical characteristics, CMC has the ability to form a water-soluble interfacial film around the cellulose micro-fibrils that allows the recovery of rheological properties after redispersion of dried samples that provide a new direction to use this redispersible dispersion in different food applications. Among the different beneficial properties of MCC, the stability of foams or emulsions is one of the most important properties. NCC produced after acidic hydrolysis of MCC can be an effective, efficient and suitable stabilizer for oil in water Pickering emulsions. It might be an efficient stabilizer for high internal phase Pickering emulsions (HIPES) and O/W Pickering emulsions systems [21–23]. Scientifically, the phenomenon of Pickering systems considered fascinating and safer and it depends on the particle size, particle shape and wettability [21]. Cellulose-based Pickering emulsions conceived as a promising alternative for surfactant based systems on account of their nontoxicity, safety, biodegradability, and sustainability. Hence, the modified MCC redispersible dispersion can be used for the preparation of Pickering emulsions as there exist limited literature on the utilization of redispersed dispersion in food and other allied sectors. According to best of author knowledge, such kind of research is first of its own kind. The limelight and aim of this study were to prepare modified, redispersible microcrystal cellulose and its application in the fabrication of Pickering emulsions. In this manuscript, we focused on investigating the impact of the molecular weight of CMCNa on the physicochemical properties of modified MCC and its role in stabilization of emulsions after re-dispersion. 2. Materials and methods 2.1. Materials Microcrystalline cellulose (MCC, α-cellulose with particle size 25 μm and molecular weight of 162.06), Sodium carboxy methyl cellulose (CMCNa) with different molecular weights [Low molecular weight (CMCNa-a, Mw 90,000); medium molecular weight (CMCNa-b 250,000) and high molecular weight (CMCNa-c 350,000)] was purchased from Aladdin Reagent Co., Ltd., Shanghai, China. All chemicals were used as received without further purification. De-ionized water was used throughout the experiment. 2.2. Preparation of CMCNa modified MCC composites The solution mixing technique was used to prepare the modified composites of MCC with CMCNa. At first, the fresh MCC dispersion (2%) was prepared in de-ionized water under continuous magnetic stirring followed by sonication (FS-600N, Shengxi Ltd., China) for 5 min with 70% amplitude. CMCNa solution (4%) was prepared in deionized water at 90 °C under continuous magnetic stirring for 6 h at 500 rpm. Then, different amounts of MCC and CMCNa combined (50:50; 60:40 and 70:30, v/v), respectively and mixed under continuous magnetic stirring, followed by homogenization with high-speed disperser (IKA-T25, at 10000 rpm for 5 min).The prepared composites (15 mL) were poured into 20 mL glass bottles and placed in the laboratory at room temperature
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to investigate the stability. Based on visual inspection, the stable composites were selected and freeze-dried at −81 °C for 48 h. The obtained dried composites were kept in a desiccator before used. 2.3. Characterizations of CMCNa modified MCC composites The particle size of unmodified MCC and modified MCC was analyzed using dynamic laser diffraction (LD) device (Mastersizer 2000, Malvern Instruments, Worcestershire, UK). Solution (0.1%) was prepared for each composite to measure the particle size. The results were reported as the average of three consecutive measurements. The morphological characterizations of unmodified MCC and its modified composites were observed with the optical microscope (SOPTOP, 0.65×). The homogenized and fine droplet of the composite was poured onto a glass slide, covered with coverslip and visualized under a lens with the magnification of 10×. The morphology was imaged with a digital microscope equipped with a CCD camera (Axio Cam HRC, Zeiss). Fourier transform infrared (FT-IR) analysis of the MCC, CMCNa and its modified composite was carried out with a FT-IR analyzer (470Nexus, Nicolet, USA) at 25 °C [24]. The samples were sufficiently dried, ground, mixed with KBr (1:100; weight ratio), and pressed into a pellet at 25 Pa. The spectra were acquired in the range of 400–4000 cm−1 at a resolution of 4 cm−1 to figure out the possible interactions between MCCs. The Zeta potential of MCC and its composite was measured by using electrophoretic light scattering device (Nano Zetasizer, Malvern Instruments, Worcestershire, UK). Solution (0.01%) was prepared with deionized water under continuous magnetic stirring at 25 °C and pH (3.0–9.0) but here presented only at pH 7. The results were reported as the average of three consecutive measurements. The rheological analysis was carried out with the rotational rheometer (Discovery Hybrid Rheometer DHR-2, USA) equipped with the steel plate geometry (40 mm) at 25 °C [25]. Measurement gap was set to 0.5 mm. The sample was held for 3 min at 25 °C to eliminate residual and thermal histories. The steady state shear rate was determined in the range of 0.1 to 1000 s−1 shear rate. The frequency sweep test was carried out in the range of 0.1 to 100 rad/s angular frequency at 0.1% strain. The rheological tests were repeated three times at same conditions to attain reproducible results. The samples were covered with silicon oil to prevent dehydration. On the basis of the visual inspection for 30 days, the composites (50:50) were freeze-dried. Then freeze-dried composites were cut into small pieces and dissolved in deionized water under continuous magnetic stirring. Then RD dispersions (2%, w/v) were prepared and 10 mL of each sample was added into 20 mL glass bottles and placed at room temperature for 30 days. Their pictures were taken at different time intervals. 2.4. Preparation of Pickering emulsions The RD dispersion (0.5–2% w/v in water) was used to prepare oil in water (O/W) emulsions. The O/W emulsions were prepared by controlling the oil/water phase ratio of 10/90, (v/v) after a preliminary experiment. Dodecane oil was selected as the oil phase. The solution was homogenized prior to preparing an emulsion. The O/W phase mixed at 4 °C with ultra turrax mixture at 12000 rpm for 3 min. Then 8 mL of the fresh emulsion was poured into 10 mL glass bottles and placed at room temperature for 15 days. 2.5. Characterization of Pickering emulsions The particle size of emulsions was measured with Mastersizer (APA2000, Malvern Ltd., UK). The mean particle size of the emulsion was taken to be the surface weighted mean diameter D (3, 2), which was calculated by using the following equation: Dð3;2Þ ¼
X
Ni D3i =
X
Ni D2i
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where Ni was the number of droplets of particles and Di was the droplets diameter. The microstructure of emulsion was visualized under an optical microscope. The Rheological behavior of emulsions prepared with 2% MCCs modified dispersion was carried out with the rotational rheometer (Discovery Hybrid Rheometer DHR-2, USA) equipped with the steel plate geometry (40 mm) at 25 °C. The steady state shear rate was determined in the range of 0.1 to 1000 s−1 shear rate. The frequency sweep test was carried out in the range of 0.1 to 100 rad/s angular frequency at 0.1% strain. The creaming index was calculated by using the following equation: CI ¼ Hs=He 100% where He was the height of total emulsion and Hs was the height of the serum layer determined at 1st and 7th day of storage. The effect of pH (3, 5, 7, 9, and 11) and ionic strength (0, 0.01, 0.05 and 0.1 M at neutral pH) on the stability of emulsions (1.5% RD dispersion) was also investigated. The Pickering emulsions prepared with 1.5% MCC modified dispersion were freeze dried. The re-dispersibility of dried emulsions was investigated. 3. Results and discussion The present study attempts at modifying the surface of MCC with the application of polymer adsorption to prevent hornification and to attain re-dispersibility. This experiment was aimed to fabricate novel Pickering emulsions with the use of re-dispersible modified MCCs. 3.1. Stability and characterization of CMCNa modified MCC composites The dynamic laser diffraction (LD) analysis expressed that surface modification altered the particle size of modified MCC. The particle size of unmodified MCC in current work was 14.208 ± 0.064 μm. The results provided a clear insight that the modified MCC with a higher molecular weight (CMCNa-c) had the highest particle size (19.576 ± 0.26 μm) followed by CMCNa-b and CMCNa-a. The smallest size (15.407 ± 0.016 μm) was determined from the composite consisting of MCC and CMCNa-a (70:30). The molecular weight and viscosity imparted a positive role in the increment of particle size. The same trend reported in a recent study that Octenyl Succinic Anhydride (OSA) increased the particle size of NCC [23]. The particle size was significantly affected by the amount of used MCC and CMCNa. The particle association between MCC and CMCNa composites was further elaborated with the optical microscopy. The MCC dispersion without CMCNa resulted in a large number of microfibrillar aggregates as shown in Fig. 1a. The addition of CMCNa provided the surface coverage to the MCC (Fig. 1b–d) and decreased the aggregates of MCC.
Furthermore, the addition of high molecular weight CMCNa increased the surface coverage of MCC through surrounding the MCC fibers in aqueous composites. In addition, Fig. 1e–g showed that the loading of CMCNa on the surface of MCC could decrease the agglomeration of MCC after re-dispersion as compared to that of freshly prepared composites. The effect of molecular weight, in this case, depicted the same trend that the more stable and uniform distribution of particles was observed with high molecular weight CMCNa. In order to evaluate the surface modification of the MCC, the functional properties of MCC, CMCNa and modified MCC composites were investigated. It was quite interesting to mention that the new absorption peak was not observed during the modification process. The no peak shift phenomenon confirmed the surface modification of MCC. The spectra of native MCC, expressed different absorption band such as band at 3344 cm−1 (due to stretching frequency of the \\OH group), 2899 cm−1 (due to C\\H stretching vibration), band at 1644 cm−1 (due to bound water absorption), 1429 cm−1 (due to \\CH2 scissoring), as it was shown in Fig. 2. Compared with the spectrum of MCC, peaks at approximately 3405 cm−1, corresponding to the OH band of cellulose, increased with the addition of CMCNa that might be due to increase hydrophilicity of the modified MCC. The peaks at 2904, 1422, 1113 and 710 cm−1 were attributed to symmetric C\\H stretching vibrations, asymmetric C\\H stretching vibrations, symmetric C\\H stretching deformation vibrations and C\\O\\C stretching vibrations, respectively. In addition, the peak at 1600 cm−1 considered as the most important peak where adsorption of \\COO band occurred which shifted from 1644 to 1600 cm−1 due to carboxymethylation which confirmed the substitution of carboxymethyl groups in MCC [15,23]. These results clearly indicated that CMCNa polymer successfully incorporated on the surface of MCC. The presence of this band in freeze-dried composites confirmed the carboxymethylation that helps in seizing the properties of redispersion. The results depicted in Fig. 3 provided a clear insight that the zeta potential increased obviously with the polymer adsorption. Zeta potential of the native MCC was −24.10 ± 0.25. The viscosity and molecular weight had a significant impact on the modified MCC and adsorption of CMC lead to significant increase in zeta potential [27]. The statistical analysis revealed that the molecular weight of the polymer, as well as the different ratios of used polymers, had a significant effect on the zeta potential of prepared composites as mentioned in Fig. 3. The composites prepared with CMCNa-a possessed lower zeta potential than CMCNa-b and CMCNa-c. The addition of high molecular weight CMC imparts significant difference as compared to CMCNa-a. The reason behind this significant difference might be carboxylation and irreversible adsorption of CMCNa onto MCC. Both materials had a negative charge on their surface and introduction of carbonyl carbon increased their surface charge [23].
Fig. 1. Digital micrographs of freshly prepared CMCNa modified MCC composites (a: MCC; b: MCC-CMC-a; c: MCC-CMC-b and d: MCC-CMC-c) and after re-dispersion of modified MCC composites (e: MCC-CMC-a; f: MCC-CMC-b and g: MCC-CMC-c). The scale bar shown correspond to 90 μm.
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e 2911
Transmittance (a.u.)
d
1601
3411
2909
c
1601
3407 2906
b a
1601
3404
2922
1604
3435
1644 2900 3344 4000 3500 3000 2500 2000 1500 1000
500
-1
Wavenumbers (cm ) Fig. 2. FT-IR spectra of unmodified MCC (a), CMCNa (b) and modified MCC composites MCC-CMCNa-a (c), MCC-CMCNa-b (d) and MCC-CMCNa-c (e).
The effect of molecular weight on the rheological behavior of modified composites (MCC: CMCNa 50:50) was carried out, and the results were shown in Fig. 4. The flat plate (40 mm) was selected to discover the flow behavior of composites. The MCC: CMCNa (50:50) composites
-70
h
g f
Zeta Potentail (mV)
-60 b
-50
e
e
e c
d
-40 -30
a
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exhibited shear thinning behavior, in this type of flow of liquid, as the shear rate increased, the viscosity of the composite decreased due to the breakdown of interlocked bonding which led to more shear thinning behavior [19,29].The viscosity increased as the polymer of high molecular weight was used. However, the composites (50:50) prepared with CMCNa-c and CMCNa-b expressed the non-newtonian, pseudoplastic and shear thinning behavior [28]. While, composite (50:50) prepared with CMCNa-a expressed the shear thinning behavior at low shear rates, which was shifted to Newtonian flow as the shear rate increased, due to weak adsorption of active polymer and weak interlocked bonding. Fig. 4b shows the viscoelastic measurements for the CMCNa modified MCC composites. It could be observed that modified composites possessed slightly gel-like behaviors as the G′ was dominated over G″ at a higher frequency [30]. However, the dynamic viscoelastic for the composites prepared with low molecular weight polymer at same ratios expressed liquid-like, non-linear flow as the G″ N G′. At the composite ratio of 60:40 (MCC: CMCNa), only composites prepared with CMCNa-c expressed the viscoelastic behavior as the G′ N G″. While composites (70:30) expressed non-linear and liquid-like flow behavior and in this case G″ N G′. The reason behind this behavior was the used amount of MCC that resulted in sedimentation, poor interlocked bonding, un-stability and weak adsorption of the polymer [25,31]. The physical stability of the composites was inspected visually with the help of digital photos. The photos taken at different time duration (1–30 days) depict that composites prepared with CMCNa-c were more stable than other types of CMC. The photos enlightened the stability, sedimentation and phase separation of the composites Fig. 5. The adsorption of CMCNa onto MCC was unfavorable due to same charge repulsion mechanism, but irreversible adsorption of active polymer with heat-induced method disappeared the charge repulsion and strengthened the irreversible adsorption due to the surface saturation of MCC with negative charge introduced by CMCNa [13]. In Addition, adding monovalent cation (Na+) and drying the resulting composite would produce Na+ − −MCC which should limit the hydrogen bonding and made the re-dispersion of dried MCC composites easily [32,33].The re-dispersibility of CMCNa modified MCCs was qualitatively assessed with sedimentation test [26].The digital images are shown in Fig. 5 under the heading of re-dispersed. The freeze-dried composite broken down into pieces, re-dispersed in water, homogenous and stable hydrogel was prepared. It was found that all composites attained the redispersibility without application of high shearing force in initial days. However, the dispersed of CMCNa-a precipitated after 3 days and the reasons behind this behavior are less adsorption and poor interlock network between polymeric suspensions. The RD dispersions with CMCNa-b and CMCNa-c attained re-dispersibility and remained stable during storage interval (7 days). No sedimentation and precipitation was observed for the MCC composites prepared with CMCNa-c during whole storage interval time. In comparison, the results enlightened that the re-dispersibility was depended on the adsorbed amount of polymer having a high molecular weight (CMCNa-c). 3.2. Stability and characterization of Pickering emulsions
-20
MC C
0
CM Ca5 0 CM Cb5 0 CM Cc5 0 CM Ca4 0 CM Cb4 0 CM Cc4 0 CM Ca3 0 CM Cb3 0 CM Cc3 0
-10
Modified Composites of MCC Fig. 3. Zeta Potential of freshly prepared unmodified MCC and CMCNa modified MCC composites. 0.1% solution was used to determine z-potential. Data presented are the means of triplicates, n = 3. Mean values having different superscript letters in a column for each sample are significantly different at P ≤ 0.05.
The impact of modified MCC composite on the fabrication of Pickering emulsions was investigated. Different concentrations (0.5 to 2%) of RD dispersion were used to prepare O/W emulsions. The unstable behavior of the emulsion system was observed at a low content. The stable emulsion system was produced at higher content of RD dispersion. Based on our systematic experiment, it was found that lower content of RD dispersions should be set to 1% or higher to prepare the stable emulsions. The emulsions prepared with higher content enabled the adsorption of more modified MCC particles at the O/W interface, which resulted in the reduction of surface tension. The emulsion prepared with low concentration (0.5–1%) expressed the creaming behavior within 6–12 h. The reason behind this behavior was that the
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Fig. 4. Viscosity as the function of shear rate for the CMCNa modified MCC composites (a) and emulsions (c) with influence of different molecular weight and viscosity and Storage modulus (G′, filled symbols) and loss modulus (G″ open symbols) as a function of angular frequency for composites (b) and Pickering emulsions (d). For better visualization filled and open symbols were used.
oil droplets were more stable than the continuous phase, so they separated the serum and cream layers very fast. The droplet size decreased and stability of emulsions increased when the content of RD composites increased (1–2%with fixed oil phase), forming clusters of micro drops and resulted in gel type emulsion systems, which is a useful quality for application of Pickering emulsions [34]. The rheological behavior of the emulsions prepared with 2% RD dispersion was investigated. The results exhibited that emulsions possessed shear-thinning rheological behavior with a reduction in viscosity
at higher shear rate Fig. 4c. The viscosity of the emulsion was increased with the use of RD dispersion of high molecular weight. The viscoelastic characterization of emulsions expressed the gel-like behavior for emulsion prepared with different molecular weight based RD dispersion as the G′ N G″ Fig. 4d. Furthermore, both moduli increased as the RD dispersion of high molecular weight used for stabilizing emulsions. The particle size of the emulsions was measured at 1st and 7th day of storage, and the results were shown in Fig. 6. It was obvious in results that as the concentration increased from 0.5 to 2%, the droplet size
Fig. 5. Sedimentation and stability of CMCNa modified MCC composites and their re-dispersions after drying. All the samples were stored at ambient conditions during storage period.
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Fig. 6. Particle size determination of emulsions prepared with different concentrations of MCC modified re-dispersed dispersion (a = 1 day; b = 7 day) and Creaming index (c = 1 day; d = 7 day) of emulsions prepared at normal conditions. Data presented are the means of triplicates, n = 3. Mean values having different superscript letters in a column for each sample are significantly different at P ≤ 0.05.
decreased. The decrease in average droplet size indicated that the larger amounts of RD particles were adsorbed at O/W interface which protected the emulsion droplets against coalescence. Moreover, the RD content had a significant effect on the formation of the threedimensional network by inhibiting the free movement of emulsion droplets and resulting in stable emulsion systems. Thus, the emulsion had small droplet size and increased stability. However, the storage time had a slight effect on size increment, as observed at the 7th day of storage. The abrupt increase in size observed with low concentration (0.5%) due to oiling off at 7th day. The creaming behavior of prepared
emulsions was inspected visually with the help of digital pictures. The creaming index had a negative relationship with creaming behavior. As the CI increased, creaming behavior decreased.CI increased and emulsified phase volume decreased with storage time [21,35]. The statistical analysis revealed that the molecular weight of the polymer, as well as the different content of RD composites, had a highly significant effect on the CI and Particle size of emulsion systems as presented in Fig. 6. The CI has clearly enlightened the effect of molecular weight and their concentration used for fabricating emulsions. The emulsified phase volume was different with the use of same content of modified
Fig. 7. Optical micrographs of the Pickering emulsions prepared with different concentration of CMCNa modified MCC re-dispersible dispersion with constant oil fraction. All micrographs were obtained within hour. The scale bar shown correspond to 20 μm.
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RD dispersion of different molecular weight; which enlightened the effect of molecular weight and viscosity in the stabilization of emulsions. The stable emulsions shifting towards flocculation type emulsions with the passage of time. The emulsions prepared with high molecular weight possessed low CI and more creaming behavior. The CI was 0 for the emulsion prepared with 2% CMCNa-c at 1st day of storage. The microstructures of emulsions were investigated, and the results were shown in Fig. 7. All droplets expressed the spherical, round shape. The droplet size gradually decreased and uniformity observed as the concentration increased. In comparison to CMCNa-c, the emulsions prepared with CMCNa-b were less stable, while it presented the same trend similar to that of CMCNa-c. Low stability and oiling off behavior of the emulsions prepared with a lower concentration (0.5% CMCNa-a) create difficulty to observe its microstructure. The effect of molecular weight on the emulsion stability was distinguished. The emulsions prepared with low molecular weight exhibited oil separation at the 3rd day of storage with low concentrations (0.5–1%), and at 7th day with higher concentration (1.5–2%). The trend was slightly different for CMCNa-b fabricated emulsions as the oil separation was observed at the 7th day of storage for low concentrations. The emulsions prepared with high molecular weight modified MCC dispersion possessed higher viscosity and stability than other types of CMC. No oil separation and coalescence was observed even at 15th day of storage. Oil particles enwrapped with modified MCC particles rose to the upper layer due to the density difference. It was of great interest to mention that no sedimentation was observed in the prepared emulsions during storage time, as it was shown in Fig. 8. This was a positive indication that MCC modified with
CMCNa has very good re-dispersibility after drying and could be used in other food applications. The effect of pH on the stability of emulsions had been investigated in terms of particle size, and creaming index of emulsions containing 1.5% wt of different RD composites. O/W emulsions were prepared at different pH values (3 to 11). It was observed that the O/W emulsions were affected significantly with pH in terms of stability, particle size and creaming index Fig. 9. The acidic pH, especially for the emulsions prepared with low and medium molecular weight CMCNa, resulted in breaking of emulsion. Specifically, the emulsions prepared with CMCNa-a, at different pH values were less stable than CMCNa-b and CMCNa-c. In addition, the emulsions prepared with CMCNa-c, at different pH values were more stable. Furthermore, the influence of pH on creaming behavior and particle size was also characterized. Under the influence of acidic pH (3, 5) particle size was larger than the emulsions prepared at basic pH conditions (7 to 11 pH). In addition, CI was higher at acidic pH and lower at basic pH conditions. However, it was of great interest to mention here that, all emulsions were stable with a different type of RD content and remained ultra-stable at neutral pH with smaller droplet size and lower CI [36]. Similarly, the effect of ionic strength on the stability of Pickering emulsions indicated that the salt concentrations affected the emulsions. The CI and size clearly distinguished the effect of salt concentration on the emulsions prepared with RD dispersions. The CI and droplet size decreased and stable emulsions were obtained as the salt concentration and molecular weight increased. However, the oil separation was observed with CMCNa-a at all used concentrations of NaCl. The dried emulsions instead of liquid emulsions provided great ease to use. The results depicted that the dried emulsions, dispersed into the water
Fig. 8. Storage stability of emulsions prepared with different concentrations of the re-dispersible dispersion of modified MCC composites. All the samples were stored at ambient conditions during storage period of 15 days.
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Fig. 9. Effect of ionic strength (a: 7th day) and pH (b: 30th day) on the stability of emulsions prepared with the re-dispersible dispersion of MCC composites. All samples were stored at ambient conditions during storage period.
without application of high shearing, attained re-dispersibility. The reason was justified as the complexation between MCC, CMCNa, and dodecane. However, creaming, stability and particle size of oil powder decreased as compared to never dried emulsions. 4. Conclusions Modified MCCs were prepared by surface modification through polymer adsorption without using any organic or harmful solvents. The particle size, FT-IR and optical microscopy of the prepared composites were confirmed the surface modification. The structural characterizations of composites revealed an increase in physical size with the utilization of polymer of high molecular weight. The rheological results revealed that the prepared MCC composites and emulsions had a typical shear thinning behavior. Modified MCC composites and Pickering emulsions prepared with RD dispersion of CMCNa-c were ultra-stable. The above results indicated that a strong inter-droplet network formation took place in emulsions. In addition, the re-dispersible dispersion played a great role to fabricate emulsions, to the best of our knowledge, this was the only work reported yet to use RD dispersion to fabricate emulsions. The results of this project concluded that the active polymer with different molecular weight had a significant influence on the composites and O/W interface. The polymer adsorption of CMCNa on MCC provided an important and green method for modification of the MCC for its potential utilization in the fabrication of eco-friendly Pickering emulsions, which could further be used in different food items as well as in other allied sectors related to food ingredients, drugs, and cosmetics. Conflict of interest The authors declare no competing financial interest. Acknowledgments This work was supported by the Wuhan Youth Science and Technology Plan (2016070204010096), and the project of the Fundamental Research Funds for the Central Universities (2662018PY060). References [1] K. Missoum, M.N. Belgacem, J. Bras, Nanofibrillated cellulose surface modification: A review, Materials 6 (5) (2013) 1745–1766.
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Contents lists available at ScienceDirect
International Journal of Biological Macromolecules journal homepage: http://www.elsevier.com/locate/ijbiomac
Surface modification of microcrystalline cellulose: Physicochemical characterization and applications in the Stabilization of Pickering emulsions Hafiz Muhammad Ahsan, Xingzhong Zhang, Yan Li, Bin Li, Shilin Liu ⁎ College of Food Science & Technology, Huazhong Agricultural University, Wuhan, Hubei 430070, China
a r t i c l e
i n f o
Article history: Received 22 October 2018 Received in revised form 12 February 2019 Accepted 8 April 2019 Available online 9 April 2019 Keywords: MCC CMCNa Composites Molecular weight Pickering emulsions
a b s t r a c t The phenomenon of polymer adsorption was applied to modify the surface of microcrystalline cellulose (MCC). The active polymer sodium carboxymethyl cellulose (CMCNa), having different molecular weight was used to produce modified MCCs. The average particle size of unmodified MCC was about 14.208 ± 0.064 μm, and it was increased to 19.576 ± 0.26 μm after modification. The modified MCCs exhibited a typical shear thinning behavior. It suggested that the molecular weight of active polymer had an amenable and significant influence on the physicochemical properties and stability of MCCs. The composites prepared from CMCNa with high molecular weight were more stable than others. Moreover, dried MCCs could be re-dispersed in water and could be used as a stabilizer for Pickering emulsions. The obtained emulsions (CMCNa-c) showed higher stability against pH changes, ionic strength and coalescence during storage. It suggested that the re-dispersible MCCs played a significant role in stabilization of emulsions. Furthermore, the re-dispersible MCCs preserved the original properties of un-dried composites and could also be used in other food allied sectors. © 2019 Elsevier B.V. All rights reserved.
1. Introduction The phenomenon of surface modification by physical or chemical means has attracted great interests in the scientific environment due to the introduction of new possible interactions in various food ingredients. Currently, the well-established processes of surface modification includes: oxidation, acid hydrolysis, esterification, amidation, nucleophilic substitution, polymer adsorption or grafting, and so on. Interestingly, each type of surface modification had its own significance and functions that entirely depends on the core theme of application [1,2]. In the recent past, this phenomenon vigorously implemented on different materials especially on cellulosic material e.g. microcrystalline cellulose (MCC) and nano-crystalline cellulose (NCC). The selection of cellulose is mainly due to its renewable nature, large specific surface area, lightweight, low cost, and unique physicochemical properties. MCC is the most commonly used cellulose and commercially available in powder form that possesses relatively high crystallinity. Normally, MCC is produced after a mechanical disintegration of cellulosic fiber pulp through a high-pressure homogenizer [3,4]. It consists of nanocrystals (NCC), obtained from hydrolysis of cellulose fibers, either in form of parts ⁎ Corresponding author. E-mail addresses: [email protected] (H.M. Ahsan), [email protected] (Y. Li), [email protected] (B. Li), [email protected] (S. Liu).
https://doi.org/10.1016/j.ijbiomac.2019.04.051 0141-8130/© 2019 Elsevier B.V. All rights reserved.
or aggregates with the diameter of several micrometers or tens of micrometers [5]. MCC, having a different particle size and micro-fibrils networks, can be produced by using different pressure, temperature, flow rate and design of chambers of high-pressure homogenizers. It has promising applications in bio-composites, protein immobilization, drug delivery, and metallic reaction templates [6,7]. The modification of cellulosic material (NCC and MCC) has been a subject of intense research in polymer composites, such as protein, chitosan, methyl cellulose, wheat bran, and corn starch [8,9]. Over the last few years, many efforts have been performed to the employment of natural cellulose fibers as reinforcing agents in polymer composites [10]. The main reason for such modificiation is the preparation of composites that can enhance the properties of both polymers [11]. A wide variety of reactions centered on imparting new properties on the surface of cellulosic materials (MCC and NCC) along with improving the compatibility of cellulose nano-crystals with matrices for the formation of composite materials [12]. MCC can be modified with homogenization as it releases the microfibrils in the suspension or through drying. However, it leads to the formation of agglomerates and bundles in MCC due to hydrogen bonding. These structural attributes after drying create an issue to re-disperse MCC to form a homogenous suspension. The whole mechanism in this process is technically responsible for hornification or irreversible agglomeration. It is one of the major issues faced in papermaking industry
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as it is difficult to re-disperse the MCC as it forms agglomerates after drying [13,14]. To eradicate this issue, many studies were conducted and surface modification was considered as a management remedy. The phenomenon of surface modification through polymer adsorption or polymer grafting considered green technology to overcome hornification and various studies were conducted to use polymers or salts to overcome hornification [13–18]. Carboxymethyl cellulose (CMC) has the potential to act as a linker molecule when adsorbed on cellulose [14]. CMC was used in different studies because of its chemical and physical characteristics; it possesses carboxyl groups [15,16]. The adsorbed CMC improved the mechanical properties of hand-sheets prepared from wood pulp and aided in paper recycling [19]. Moreover, CMC adsorbed onto cotton fibers could increase the charge density and adsorption capacity for surfactant [20]. Owing to various physicochemical characteristics, CMC has the ability to form a water-soluble interfacial film around the cellulose micro-fibrils that allows the recovery of rheological properties after redispersion of dried samples that provide a new direction to use this redispersible dispersion in different food applications. Among the different beneficial properties of MCC, the stability of foams or emulsions is one of the most important properties. NCC produced after acidic hydrolysis of MCC can be an effective, efficient and suitable stabilizer for oil in water Pickering emulsions. It might be an efficient stabilizer for high internal phase Pickering emulsions (HIPES) and O/W Pickering emulsions systems [21–23]. Scientifically, the phenomenon of Pickering systems considered fascinating and safer and it depends on the particle size, particle shape and wettability [21]. Cellulose-based Pickering emulsions conceived as a promising alternative for surfactant based systems on account of their nontoxicity, safety, biodegradability, and sustainability. Hence, the modified MCC redispersible dispersion can be used for the preparation of Pickering emulsions as there exist limited literature on the utilization of redispersed dispersion in food and other allied sectors. According to best of author knowledge, such kind of research is first of its own kind. The limelight and aim of this study were to prepare modified, redispersible microcrystal cellulose and its application in the fabrication of Pickering emulsions. In this manuscript, we focused on investigating the impact of the molecular weight of CMCNa on the physicochemical properties of modified MCC and its role in stabilization of emulsions after re-dispersion. 2. Materials and methods 2.1. Materials Microcrystalline cellulose (MCC, α-cellulose with particle size 25 μm and molecular weight of 162.06), Sodium carboxy methyl cellulose (CMCNa) with different molecular weights [Low molecular weight (CMCNa-a, Mw 90,000); medium molecular weight (CMCNa-b 250,000) and high molecular weight (CMCNa-c 350,000)] was purchased from Aladdin Reagent Co., Ltd., Shanghai, China. All chemicals were used as received without further purification. De-ionized water was used throughout the experiment. 2.2. Preparation of CMCNa modified MCC composites The solution mixing technique was used to prepare the modified composites of MCC with CMCNa. At first, the fresh MCC dispersion (2%) was prepared in de-ionized water under continuous magnetic stirring followed by sonication (FS-600N, Shengxi Ltd., China) for 5 min with 70% amplitude. CMCNa solution (4%) was prepared in deionized water at 90 °C under continuous magnetic stirring for 6 h at 500 rpm. Then, different amounts of MCC and CMCNa combined (50:50; 60:40 and 70:30, v/v), respectively and mixed under continuous magnetic stirring, followed by homogenization with high-speed disperser (IKA-T25, at 10000 rpm for 5 min).The prepared composites (15 mL) were poured into 20 mL glass bottles and placed in the laboratory at room temperature
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to investigate the stability. Based on visual inspection, the stable composites were selected and freeze-dried at −81 °C for 48 h. The obtained dried composites were kept in a desiccator before used. 2.3. Characterizations of CMCNa modified MCC composites The particle size of unmodified MCC and modified MCC was analyzed using dynamic laser diffraction (LD) device (Mastersizer 2000, Malvern Instruments, Worcestershire, UK). Solution (0.1%) was prepared for each composite to measure the particle size. The results were reported as the average of three consecutive measurements. The morphological characterizations of unmodified MCC and its modified composites were observed with the optical microscope (SOPTOP, 0.65×). The homogenized and fine droplet of the composite was poured onto a glass slide, covered with coverslip and visualized under a lens with the magnification of 10×. The morphology was imaged with a digital microscope equipped with a CCD camera (Axio Cam HRC, Zeiss). Fourier transform infrared (FT-IR) analysis of the MCC, CMCNa and its modified composite was carried out with a FT-IR analyzer (470Nexus, Nicolet, USA) at 25 °C [24]. The samples were sufficiently dried, ground, mixed with KBr (1:100; weight ratio), and pressed into a pellet at 25 Pa. The spectra were acquired in the range of 400–4000 cm−1 at a resolution of 4 cm−1 to figure out the possible interactions between MCCs. The Zeta potential of MCC and its composite was measured by using electrophoretic light scattering device (Nano Zetasizer, Malvern Instruments, Worcestershire, UK). Solution (0.01%) was prepared with deionized water under continuous magnetic stirring at 25 °C and pH (3.0–9.0) but here presented only at pH 7. The results were reported as the average of three consecutive measurements. The rheological analysis was carried out with the rotational rheometer (Discovery Hybrid Rheometer DHR-2, USA) equipped with the steel plate geometry (40 mm) at 25 °C [25]. Measurement gap was set to 0.5 mm. The sample was held for 3 min at 25 °C to eliminate residual and thermal histories. The steady state shear rate was determined in the range of 0.1 to 1000 s−1 shear rate. The frequency sweep test was carried out in the range of 0.1 to 100 rad/s angular frequency at 0.1% strain. The rheological tests were repeated three times at same conditions to attain reproducible results. The samples were covered with silicon oil to prevent dehydration. On the basis of the visual inspection for 30 days, the composites (50:50) were freeze-dried. Then freeze-dried composites were cut into small pieces and dissolved in deionized water under continuous magnetic stirring. Then RD dispersions (2%, w/v) were prepared and 10 mL of each sample was added into 20 mL glass bottles and placed at room temperature for 30 days. Their pictures were taken at different time intervals. 2.4. Preparation of Pickering emulsions The RD dispersion (0.5–2% w/v in water) was used to prepare oil in water (O/W) emulsions. The O/W emulsions were prepared by controlling the oil/water phase ratio of 10/90, (v/v) after a preliminary experiment. Dodecane oil was selected as the oil phase. The solution was homogenized prior to preparing an emulsion. The O/W phase mixed at 4 °C with ultra turrax mixture at 12000 rpm for 3 min. Then 8 mL of the fresh emulsion was poured into 10 mL glass bottles and placed at room temperature for 15 days. 2.5. Characterization of Pickering emulsions The particle size of emulsions was measured with Mastersizer (APA2000, Malvern Ltd., UK). The mean particle size of the emulsion was taken to be the surface weighted mean diameter D (3, 2), which was calculated by using the following equation: Dð3;2Þ ¼
X
Ni D3i =
X
Ni D2i
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where Ni was the number of droplets of particles and Di was the droplets diameter. The microstructure of emulsion was visualized under an optical microscope. The Rheological behavior of emulsions prepared with 2% MCCs modified dispersion was carried out with the rotational rheometer (Discovery Hybrid Rheometer DHR-2, USA) equipped with the steel plate geometry (40 mm) at 25 °C. The steady state shear rate was determined in the range of 0.1 to 1000 s−1 shear rate. The frequency sweep test was carried out in the range of 0.1 to 100 rad/s angular frequency at 0.1% strain. The creaming index was calculated by using the following equation: CI ¼ Hs=He 100% where He was the height of total emulsion and Hs was the height of the serum layer determined at 1st and 7th day of storage. The effect of pH (3, 5, 7, 9, and 11) and ionic strength (0, 0.01, 0.05 and 0.1 M at neutral pH) on the stability of emulsions (1.5% RD dispersion) was also investigated. The Pickering emulsions prepared with 1.5% MCC modified dispersion were freeze dried. The re-dispersibility of dried emulsions was investigated. 3. Results and discussion The present study attempts at modifying the surface of MCC with the application of polymer adsorption to prevent hornification and to attain re-dispersibility. This experiment was aimed to fabricate novel Pickering emulsions with the use of re-dispersible modified MCCs. 3.1. Stability and characterization of CMCNa modified MCC composites The dynamic laser diffraction (LD) analysis expressed that surface modification altered the particle size of modified MCC. The particle size of unmodified MCC in current work was 14.208 ± 0.064 μm. The results provided a clear insight that the modified MCC with a higher molecular weight (CMCNa-c) had the highest particle size (19.576 ± 0.26 μm) followed by CMCNa-b and CMCNa-a. The smallest size (15.407 ± 0.016 μm) was determined from the composite consisting of MCC and CMCNa-a (70:30). The molecular weight and viscosity imparted a positive role in the increment of particle size. The same trend reported in a recent study that Octenyl Succinic Anhydride (OSA) increased the particle size of NCC [23]. The particle size was significantly affected by the amount of used MCC and CMCNa. The particle association between MCC and CMCNa composites was further elaborated with the optical microscopy. The MCC dispersion without CMCNa resulted in a large number of microfibrillar aggregates as shown in Fig. 1a. The addition of CMCNa provided the surface coverage to the MCC (Fig. 1b–d) and decreased the aggregates of MCC.
Furthermore, the addition of high molecular weight CMCNa increased the surface coverage of MCC through surrounding the MCC fibers in aqueous composites. In addition, Fig. 1e–g showed that the loading of CMCNa on the surface of MCC could decrease the agglomeration of MCC after re-dispersion as compared to that of freshly prepared composites. The effect of molecular weight, in this case, depicted the same trend that the more stable and uniform distribution of particles was observed with high molecular weight CMCNa. In order to evaluate the surface modification of the MCC, the functional properties of MCC, CMCNa and modified MCC composites were investigated. It was quite interesting to mention that the new absorption peak was not observed during the modification process. The no peak shift phenomenon confirmed the surface modification of MCC. The spectra of native MCC, expressed different absorption band such as band at 3344 cm−1 (due to stretching frequency of the \\OH group), 2899 cm−1 (due to C\\H stretching vibration), band at 1644 cm−1 (due to bound water absorption), 1429 cm−1 (due to \\CH2 scissoring), as it was shown in Fig. 2. Compared with the spectrum of MCC, peaks at approximately 3405 cm−1, corresponding to the OH band of cellulose, increased with the addition of CMCNa that might be due to increase hydrophilicity of the modified MCC. The peaks at 2904, 1422, 1113 and 710 cm−1 were attributed to symmetric C\\H stretching vibrations, asymmetric C\\H stretching vibrations, symmetric C\\H stretching deformation vibrations and C\\O\\C stretching vibrations, respectively. In addition, the peak at 1600 cm−1 considered as the most important peak where adsorption of \\COO band occurred which shifted from 1644 to 1600 cm−1 due to carboxymethylation which confirmed the substitution of carboxymethyl groups in MCC [15,23]. These results clearly indicated that CMCNa polymer successfully incorporated on the surface of MCC. The presence of this band in freeze-dried composites confirmed the carboxymethylation that helps in seizing the properties of redispersion. The results depicted in Fig. 3 provided a clear insight that the zeta potential increased obviously with the polymer adsorption. Zeta potential of the native MCC was −24.10 ± 0.25. The viscosity and molecular weight had a significant impact on the modified MCC and adsorption of CMC lead to significant increase in zeta potential [27]. The statistical analysis revealed that the molecular weight of the polymer, as well as the different ratios of used polymers, had a significant effect on the zeta potential of prepared composites as mentioned in Fig. 3. The composites prepared with CMCNa-a possessed lower zeta potential than CMCNa-b and CMCNa-c. The addition of high molecular weight CMC imparts significant difference as compared to CMCNa-a. The reason behind this significant difference might be carboxylation and irreversible adsorption of CMCNa onto MCC. Both materials had a negative charge on their surface and introduction of carbonyl carbon increased their surface charge [23].
Fig. 1. Digital micrographs of freshly prepared CMCNa modified MCC composites (a: MCC; b: MCC-CMC-a; c: MCC-CMC-b and d: MCC-CMC-c) and after re-dispersion of modified MCC composites (e: MCC-CMC-a; f: MCC-CMC-b and g: MCC-CMC-c). The scale bar shown correspond to 90 μm.
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e 2911
Transmittance (a.u.)
d
1601
3411
2909
c
1601
3407 2906
b a
1601
3404
2922
1604
3435
1644 2900 3344 4000 3500 3000 2500 2000 1500 1000
500
-1
Wavenumbers (cm ) Fig. 2. FT-IR spectra of unmodified MCC (a), CMCNa (b) and modified MCC composites MCC-CMCNa-a (c), MCC-CMCNa-b (d) and MCC-CMCNa-c (e).
The effect of molecular weight on the rheological behavior of modified composites (MCC: CMCNa 50:50) was carried out, and the results were shown in Fig. 4. The flat plate (40 mm) was selected to discover the flow behavior of composites. The MCC: CMCNa (50:50) composites
-70
h
g f
Zeta Potentail (mV)
-60 b
-50
e
e
e c
d
-40 -30
a
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exhibited shear thinning behavior, in this type of flow of liquid, as the shear rate increased, the viscosity of the composite decreased due to the breakdown of interlocked bonding which led to more shear thinning behavior [19,29].The viscosity increased as the polymer of high molecular weight was used. However, the composites (50:50) prepared with CMCNa-c and CMCNa-b expressed the non-newtonian, pseudoplastic and shear thinning behavior [28]. While, composite (50:50) prepared with CMCNa-a expressed the shear thinning behavior at low shear rates, which was shifted to Newtonian flow as the shear rate increased, due to weak adsorption of active polymer and weak interlocked bonding. Fig. 4b shows the viscoelastic measurements for the CMCNa modified MCC composites. It could be observed that modified composites possessed slightly gel-like behaviors as the G′ was dominated over G″ at a higher frequency [30]. However, the dynamic viscoelastic for the composites prepared with low molecular weight polymer at same ratios expressed liquid-like, non-linear flow as the G″ N G′. At the composite ratio of 60:40 (MCC: CMCNa), only composites prepared with CMCNa-c expressed the viscoelastic behavior as the G′ N G″. While composites (70:30) expressed non-linear and liquid-like flow behavior and in this case G″ N G′. The reason behind this behavior was the used amount of MCC that resulted in sedimentation, poor interlocked bonding, un-stability and weak adsorption of the polymer [25,31]. The physical stability of the composites was inspected visually with the help of digital photos. The photos taken at different time duration (1–30 days) depict that composites prepared with CMCNa-c were more stable than other types of CMC. The photos enlightened the stability, sedimentation and phase separation of the composites Fig. 5. The adsorption of CMCNa onto MCC was unfavorable due to same charge repulsion mechanism, but irreversible adsorption of active polymer with heat-induced method disappeared the charge repulsion and strengthened the irreversible adsorption due to the surface saturation of MCC with negative charge introduced by CMCNa [13]. In Addition, adding monovalent cation (Na+) and drying the resulting composite would produce Na+ − −MCC which should limit the hydrogen bonding and made the re-dispersion of dried MCC composites easily [32,33].The re-dispersibility of CMCNa modified MCCs was qualitatively assessed with sedimentation test [26].The digital images are shown in Fig. 5 under the heading of re-dispersed. The freeze-dried composite broken down into pieces, re-dispersed in water, homogenous and stable hydrogel was prepared. It was found that all composites attained the redispersibility without application of high shearing force in initial days. However, the dispersed of CMCNa-a precipitated after 3 days and the reasons behind this behavior are less adsorption and poor interlock network between polymeric suspensions. The RD dispersions with CMCNa-b and CMCNa-c attained re-dispersibility and remained stable during storage interval (7 days). No sedimentation and precipitation was observed for the MCC composites prepared with CMCNa-c during whole storage interval time. In comparison, the results enlightened that the re-dispersibility was depended on the adsorbed amount of polymer having a high molecular weight (CMCNa-c). 3.2. Stability and characterization of Pickering emulsions
-20
MC C
0
CM Ca5 0 CM Cb5 0 CM Cc5 0 CM Ca4 0 CM Cb4 0 CM Cc4 0 CM Ca3 0 CM Cb3 0 CM Cc3 0
-10
Modified Composites of MCC Fig. 3. Zeta Potential of freshly prepared unmodified MCC and CMCNa modified MCC composites. 0.1% solution was used to determine z-potential. Data presented are the means of triplicates, n = 3. Mean values having different superscript letters in a column for each sample are significantly different at P ≤ 0.05.
The impact of modified MCC composite on the fabrication of Pickering emulsions was investigated. Different concentrations (0.5 to 2%) of RD dispersion were used to prepare O/W emulsions. The unstable behavior of the emulsion system was observed at a low content. The stable emulsion system was produced at higher content of RD dispersion. Based on our systematic experiment, it was found that lower content of RD dispersions should be set to 1% or higher to prepare the stable emulsions. The emulsions prepared with higher content enabled the adsorption of more modified MCC particles at the O/W interface, which resulted in the reduction of surface tension. The emulsion prepared with low concentration (0.5–1%) expressed the creaming behavior within 6–12 h. The reason behind this behavior was that the
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Fig. 4. Viscosity as the function of shear rate for the CMCNa modified MCC composites (a) and emulsions (c) with influence of different molecular weight and viscosity and Storage modulus (G′, filled symbols) and loss modulus (G″ open symbols) as a function of angular frequency for composites (b) and Pickering emulsions (d). For better visualization filled and open symbols were used.
oil droplets were more stable than the continuous phase, so they separated the serum and cream layers very fast. The droplet size decreased and stability of emulsions increased when the content of RD composites increased (1–2%with fixed oil phase), forming clusters of micro drops and resulted in gel type emulsion systems, which is a useful quality for application of Pickering emulsions [34]. The rheological behavior of the emulsions prepared with 2% RD dispersion was investigated. The results exhibited that emulsions possessed shear-thinning rheological behavior with a reduction in viscosity
at higher shear rate Fig. 4c. The viscosity of the emulsion was increased with the use of RD dispersion of high molecular weight. The viscoelastic characterization of emulsions expressed the gel-like behavior for emulsion prepared with different molecular weight based RD dispersion as the G′ N G″ Fig. 4d. Furthermore, both moduli increased as the RD dispersion of high molecular weight used for stabilizing emulsions. The particle size of the emulsions was measured at 1st and 7th day of storage, and the results were shown in Fig. 6. It was obvious in results that as the concentration increased from 0.5 to 2%, the droplet size
Fig. 5. Sedimentation and stability of CMCNa modified MCC composites and their re-dispersions after drying. All the samples were stored at ambient conditions during storage period.
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Fig. 6. Particle size determination of emulsions prepared with different concentrations of MCC modified re-dispersed dispersion (a = 1 day; b = 7 day) and Creaming index (c = 1 day; d = 7 day) of emulsions prepared at normal conditions. Data presented are the means of triplicates, n = 3. Mean values having different superscript letters in a column for each sample are significantly different at P ≤ 0.05.
decreased. The decrease in average droplet size indicated that the larger amounts of RD particles were adsorbed at O/W interface which protected the emulsion droplets against coalescence. Moreover, the RD content had a significant effect on the formation of the threedimensional network by inhibiting the free movement of emulsion droplets and resulting in stable emulsion systems. Thus, the emulsion had small droplet size and increased stability. However, the storage time had a slight effect on size increment, as observed at the 7th day of storage. The abrupt increase in size observed with low concentration (0.5%) due to oiling off at 7th day. The creaming behavior of prepared
emulsions was inspected visually with the help of digital pictures. The creaming index had a negative relationship with creaming behavior. As the CI increased, creaming behavior decreased.CI increased and emulsified phase volume decreased with storage time [21,35]. The statistical analysis revealed that the molecular weight of the polymer, as well as the different content of RD composites, had a highly significant effect on the CI and Particle size of emulsion systems as presented in Fig. 6. The CI has clearly enlightened the effect of molecular weight and their concentration used for fabricating emulsions. The emulsified phase volume was different with the use of same content of modified
Fig. 7. Optical micrographs of the Pickering emulsions prepared with different concentration of CMCNa modified MCC re-dispersible dispersion with constant oil fraction. All micrographs were obtained within hour. The scale bar shown correspond to 20 μm.
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RD dispersion of different molecular weight; which enlightened the effect of molecular weight and viscosity in the stabilization of emulsions. The stable emulsions shifting towards flocculation type emulsions with the passage of time. The emulsions prepared with high molecular weight possessed low CI and more creaming behavior. The CI was 0 for the emulsion prepared with 2% CMCNa-c at 1st day of storage. The microstructures of emulsions were investigated, and the results were shown in Fig. 7. All droplets expressed the spherical, round shape. The droplet size gradually decreased and uniformity observed as the concentration increased. In comparison to CMCNa-c, the emulsions prepared with CMCNa-b were less stable, while it presented the same trend similar to that of CMCNa-c. Low stability and oiling off behavior of the emulsions prepared with a lower concentration (0.5% CMCNa-a) create difficulty to observe its microstructure. The effect of molecular weight on the emulsion stability was distinguished. The emulsions prepared with low molecular weight exhibited oil separation at the 3rd day of storage with low concentrations (0.5–1%), and at 7th day with higher concentration (1.5–2%). The trend was slightly different for CMCNa-b fabricated emulsions as the oil separation was observed at the 7th day of storage for low concentrations. The emulsions prepared with high molecular weight modified MCC dispersion possessed higher viscosity and stability than other types of CMC. No oil separation and coalescence was observed even at 15th day of storage. Oil particles enwrapped with modified MCC particles rose to the upper layer due to the density difference. It was of great interest to mention that no sedimentation was observed in the prepared emulsions during storage time, as it was shown in Fig. 8. This was a positive indication that MCC modified with
CMCNa has very good re-dispersibility after drying and could be used in other food applications. The effect of pH on the stability of emulsions had been investigated in terms of particle size, and creaming index of emulsions containing 1.5% wt of different RD composites. O/W emulsions were prepared at different pH values (3 to 11). It was observed that the O/W emulsions were affected significantly with pH in terms of stability, particle size and creaming index Fig. 9. The acidic pH, especially for the emulsions prepared with low and medium molecular weight CMCNa, resulted in breaking of emulsion. Specifically, the emulsions prepared with CMCNa-a, at different pH values were less stable than CMCNa-b and CMCNa-c. In addition, the emulsions prepared with CMCNa-c, at different pH values were more stable. Furthermore, the influence of pH on creaming behavior and particle size was also characterized. Under the influence of acidic pH (3, 5) particle size was larger than the emulsions prepared at basic pH conditions (7 to 11 pH). In addition, CI was higher at acidic pH and lower at basic pH conditions. However, it was of great interest to mention here that, all emulsions were stable with a different type of RD content and remained ultra-stable at neutral pH with smaller droplet size and lower CI [36]. Similarly, the effect of ionic strength on the stability of Pickering emulsions indicated that the salt concentrations affected the emulsions. The CI and size clearly distinguished the effect of salt concentration on the emulsions prepared with RD dispersions. The CI and droplet size decreased and stable emulsions were obtained as the salt concentration and molecular weight increased. However, the oil separation was observed with CMCNa-a at all used concentrations of NaCl. The dried emulsions instead of liquid emulsions provided great ease to use. The results depicted that the dried emulsions, dispersed into the water
Fig. 8. Storage stability of emulsions prepared with different concentrations of the re-dispersible dispersion of modified MCC composites. All the samples were stored at ambient conditions during storage period of 15 days.
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Fig. 9. Effect of ionic strength (a: 7th day) and pH (b: 30th day) on the stability of emulsions prepared with the re-dispersible dispersion of MCC composites. All samples were stored at ambient conditions during storage period.
without application of high shearing, attained re-dispersibility. The reason was justified as the complexation between MCC, CMCNa, and dodecane. However, creaming, stability and particle size of oil powder decreased as compared to never dried emulsions. 4. Conclusions Modified MCCs were prepared by surface modification through polymer adsorption without using any organic or harmful solvents. The particle size, FT-IR and optical microscopy of the prepared composites were confirmed the surface modification. The structural characterizations of composites revealed an increase in physical size with the utilization of polymer of high molecular weight. The rheological results revealed that the prepared MCC composites and emulsions had a typical shear thinning behavior. Modified MCC composites and Pickering emulsions prepared with RD dispersion of CMCNa-c were ultra-stable. The above results indicated that a strong inter-droplet network formation took place in emulsions. In addition, the re-dispersible dispersion played a great role to fabricate emulsions, to the best of our knowledge, this was the only work reported yet to use RD dispersion to fabricate emulsions. The results of this project concluded that the active polymer with different molecular weight had a significant influence on the composites and O/W interface. The polymer adsorption of CMCNa on MCC provided an important and green method for modification of the MCC for its potential utilization in the fabrication of eco-friendly Pickering emulsions, which could further be used in different food items as well as in other allied sectors related to food ingredients, drugs, and cosmetics. Conflict of interest The authors declare no competing financial interest. Acknowledgments This work was supported by the Wuhan Youth Science and Technology Plan (2016070204010096), and the project of the Fundamental Research Funds for the Central Universities (2662018PY060). References [1] K. Missoum, M.N. Belgacem, J. Bras, Nanofibrillated cellulose surface modification: A review, Materials 6 (5) (2013) 1745–1766.
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